Microwave resonant cavities find wide applications in atomic absorption frequency standards. In such applications, a gas vapor cell containing a vapor of a metal alkali, for example, rubidium 87, is placed within the microwave resonant cavity. Then the radio frequency power in the cavity excites the alkali to its resonant frequency which in turn stimulates atomic transition of the atoms in the gas.
A typical scheme for atomic absorption frequency standard is illustrated in FIG. 4. A light from a lamp assembly, a rubidium lamp in this example, passes through the excited gas vapor cell and impinges on a photocell. The light is detected by the photocell and is converted to an electrical signal, which is then amplified. The intensity of the detected light is proportional to the opacity of the gas vapor which, in turn, is predominantly dependent on the exciting frequency of the resonant cavity. Thus, by monitoring the opacity of the gas vapor, the exciting frequency of the resonator can be made substantially constant by appropriate feedback circuitry. It is this substantially constant and ultrastable frequency that furnishes the standard for the atomic frequency standard. The principles behind this type of atomic absorptive frequency standard are well known, and are well documented in the art through numerous publications, e.g., Proceedings of the IEEE, January 1963, pp. 190-202, and U.S. Pat. No. 3,798,565.
A resonant cavity for applications in atomic frequency standards ideally would have a uniform magnetic H-field which is collinear to both a biasing direct current (D.C.) magnetic C-field and the optical path defined by the lamp assembly. The H-field ideally would also be removed from the electric E-fields. By having a uniform H-field in alignment with the optical path, the opacity of the gas is substantially unaffected by the H-field. As a consequence, any variation in the opacity of the gas vapor can be attributed nearly entirely to any variation in the resonant cavity exciting frequency.
The uniform H-field should be substantially removed from the E-field so that the presence of the gas vapor cell would have a minimal effect on the resonant cavity frequency. The gas cell in the presence of a strong E-field loads the resonant cavity, thereby lowering the Q of the cavity and shifting the resonant frequency of the cavity. Furthermore, any metal alkali deposited on the glass wall of the gas vapor cell in the presence of a high E-field further loads the cavity, thereby further degrading the operation of the microwave resonant cavity.
Examples of previous designs used for such an application are shown in FIGS. 2 and 3. FIG. 2 illustrates an example of a TE011 right circular cylindrical microwave cavity; FIG. 3 shows an example of a TE111 right circular cylindrical microwave cavity made by Efratom Company and described in U.S. Pat. No. 3,798,565. Both designs, however, have disadvantages for their use in atomic absorption frequency standards.
The TE011 microwave resonant cavity shown in FIG. 2 is used in atomic frequency standards HP5065 and R20, manufactured by Hewlett-Packard Company and Varian Associates, respectively. One of the disadvantages to this resonant cavity is its relatively large size in comparison to the present invention. The volume of a resonant cavity is determined predominantly by its operating frequency; hence, in the present example of a rubidium gas cell, the required operating frequency of 6.8 Gigahertz determines the cavity size for a cavity operation in the TE011 mode. This TE011 resonant cavity also has the disadvantage of a high E-field in the region of the gas vapor cell. Consequently, the cavity is extremely sensitive to slight changes such as ambient temperature and the like.
To reduce the cavity size inherent in operating at the frequency necessary to excite the metal alkali vapor in the gas cell, one design uses the TE111 mode. This design is shown in FIG. 3. The cavity in this design is electrically loaded by incorporating a material with a high dielectric constant 30 and 32 in the cavity and by shaping the gas vapor cell 31 to the contour of the cavity to substantially fill the cavity. By contouring the gas cell, the gas cell also efficiently uses the reduced volume in the cavity caused by introducing a dielectric loading material in the cavity. See FIG. 1 of U.S. Pat. No. 3,798,565. One obvious disadvantage to this design is the special conformal shaping of the gas cell required to account for the protruding dielectric load in this resonant cavity. Such a requirement necessitates special handling and associated increased costs. Another disadvantage is the lack of a uniform H-field or a convenient H-field with which the optical axis could be aligned.